Coating catalyst



United States Patent 3,414,427 COATING CATALYST Joseph Peppo Levy, Harrow, England, assignor, by mesne assignments, to Sperry Rand Limited, London, England, a company of England No Drawing. Filed May 5, 1965, Ser. No. 453,497 Claims priority, application Great Britain, May 7, 1964, 19,002/ 64 6 Claims. (Cl. 11747) ABSTRACT OF THE DISCLOSURE A method of catalysing a surface of a material to be plated by a chemical reduction plating process. The catalysing method comprises treating the surface to be plated before immersion in the plating bath with a catalyst comprising a solution of a complex of palladium chloride dissolved in an organic solvent. Said complex comprises palladium chloride with hydrogen halide and water which is evaporated to a condition not dryer than a paste.

The invention relates to processes for plating surfaces with metals.

In chemical reduction plating processes a surface is coated with metal by immersing it in a plating bath containing a salt of the metal and a suitable reducing agent. The solution is unstable or metastable, and the surface to be plated is pretreated with a catalyst for the reduction reaction so that the metal is reduced and deposited on the treated surface, and only slowly, or not at all,- on the walls of the plating bath or homogeneously throughout the solution. An example of such a chemical reduction plating process is plating with nickel from a solution containing soluble nickel salts and hypophosphites.

Hitherto the surface has been catalysed in these processes by an aqueous solution of palladium chloride. Sometimes the catalysing step is preceded by treatment with a sensitising solution of stannous chloride, which promotes the formation of a more uniform and adherent metal deposit. Rigorous cleaning and preparation of the surfaces is, of course, required in all processes of this kind.

However, even with the use of these catalyst solutions plating may have poor adherence on metal surfaces, and often fails completely on non-metallic surfaces. The present invention utilises a catalyst solution which gives a striking improvement in this respect.

We have now found that palladium chloride will form a complex with hydrogen chloride and water and that it is this complex which is soluble in acetone, and which is responsible for the activity of the acetone solutions, as catalysts for reduction plating. We have found that by suitable methods it is possible to produce solutions of this complex in a very wide range of organic solvents, and that substantially all of these solutions show catalytic activity for the plating process on many substrates. Complexes are also formed with hydrobromic and hydriodic acids; in these cases also the presence of water is essential to the formation of the complexes and their solutions in organic solvents show catalytic activity in plating proc esses.

According to one aspect of the present invention a chemical reduction plating process includes the step of treating the surface to be plated before immersion in the plating bath with a catalyst comprising a solution of a complex of palladium chloride with hydrogen halide and water, dissolved in an organic solvent. The hydrogen halide is preferably hydrogen chloride.

The treatment is more effective if the surface to be plated is preheated, being brought before treatment to a temperature not substantially below the boiling point of the highest boiling constituent of the catalyst solvents. In this case the treatment is of the nature of a quenching step. The composition of the solution may be chosen with reference to the highest temperature at which heat treatment may be carried out without damaging or distorting the article to be plated. For example glass or metal may be treated with a solution containing relatively high boiling point solvents whereas for plastics which cannot be heated substantially above room temperature the solvent should contain more volatile constituents, for example acetaldehyde, which boils at approximately room temperature.

After immersion the surface to be plated is preferably again heated to a temperature not substantially below the boiling point of the highest boiling constituent of the catalyst solvents. The treatment with the catalyst solution is preferably repeated a number of times, being carried out, for example, from three to six times before plating.

The treatment is applicable to plating on both metals and non-metals, and in particular to plating with nickel or cobalt from a hypophosphite bath, but the treatment with the catalyst solution may be employed in any chemical reduction platin process where deposition is catalysed by the presence of palladium compounds.

Preferred concentrations of the catalyst solution are those containing up to one gram per litre of palladium chloride, and a solution containing 0.2 gram per litre has proved generally satisfactory.

Acetone is a suitable organic solvent for the palladium chloride complex in many cases, or where boiling point of acetone is not ideal for the proposed condition of heat treatment, mixtures of acetone with other substances may be employed. For treating plastic materials it is an advantage if at least one constituent of the catalyst solution is a liquid having some slight softening or etching effect on the surface of the plastic, and many organic esters having suitable boiling points are effective. A preferred composition of the catalyst solution comprises up to one gram of palladium chloride per litre converted to the complex, and dissolved in a mixture of acetone with one or more of the substances specified in Table III below. This is to 'be used on conjunction with a plastic substrate which can be safely heated to a temperature within the range indicated in that table. Preferably the mixture contains about 40% acetone and of the substance by volume.

Palladium chloride is reported in the chemical literature to exist in the form of delaquesent needles readily soluble in water. However, the commercially available form of palladium chloride is virtually insoluble in water, but is readily soluble in small quantities of concentrated hydrochloric acid.

According to another aspect of the present invention a method of preparing a solution of a palladium chloride complex in an organic solvent comprises dissolving palladium chloride in a concentrated aqueous solution of a hydrogen halide, preferably hydrogen chloride, evaporating the solution to drive off the greater part of the excess hydrogen halide and water, and dissolving the residue in an organic solvent.

Preferably evaporation is continued until the solution is concentrated to a thick syrup. This syrup, after cooling, is readily soluble in many organic solvents, including acetone, either cold, or with gentle warming. It still contains a small quantity of excess acid, and if this is objectionable and a neutral solution is required the acid may be neutralized by repeated small additions of solid sodium bicarbonate to the acetone solution. The sodium hali e formed, together with any excess sodium bi-carbonate, both of which are insoluble in acetone, are separated by filtering the. solution.

If desired, evaporation may be continued beyond the syrup stage until the complex forms a pasty mass, and driving off even more of the excess acid and water. This mass decomposes above 115 C., but even if this temperature is not exceeded some of the complex may be converted to an insoluble form, so that the mass cannot be completely dissolved in the organic solvent.

For plating certain refractory materials, including ceramics, silicon, molybdenum and tungsten, the complex may be first formed as above with hydrogen chloride and evaporated to a syrupy liquid, dissolved in a limited quantity of acetone, and then the required amount of an aqueous solution of hydrogen fluoride is added. This may constitute, for example, 5-10 percent by volume of the total bulk of catalyst solution.

A palladium chloride complex prepared as a syrup by the above method was found to be soluble in the following liquids:

Table I Solvent Chemical classification 1. Acetone Aliphatic ketone. 2. Methyl ethyl ketone Do. 3. p-Methyl acetophenone Aromatic ketone. 4. Acetaldehyde Aliphatic aldehyde. 5. Dioxan Alicyclic ether. 6. Iso-propyl alcohol Aliphatic alcohol. 7. Tetrahydropuran O-containing alicyclic. 8. Pyridine N-containing alicyclic. 9. Aniline Aromatic amine. 10. Propylene glycol Glycol. 11. Glacial acetic acid Aliphatic carboxylic acid. 12. Diethyl succinate Ester. 13. n-Butyl formate Do. 14. Iso-propyl acetate Do. 15. Ethyl benzoate Do. 16. Dimethyl formamide Amide. 17. Anethole Aromatic ether. 18. Ethyl salicylate Ester. 19. Methyl salicylate D0.

Complete solution was only obtained after warming, but the solution once formed was stable with TableII Chemical clas- Solvent sification. p-Methyl acetophenone Armoatic 'ketone. Dioxan Alicyclic ether. Iso-propyl acetate Ester. Ethyl benzoate Do.

The solutions in anethole, ethyl salicylate and methyl salicylate had a cloudy appearance such as to suggest the presence of colloidal particles.

All of these solutions were tested for their catalytic activity on copper, on smooth glass, on vapour blasted glass, and on a vapour blasted phenol formaldehyde plastic surface. Catalytic activity was found in every case, except that aniline and pyridine inhibited plating of copper, and produced 'very poor results on the plastic. Both of these substances gave solutions which were effective catalysts for plating on glass.

Solutions Were also prepared in mixtures of these various solvents, and in general it was found that mixtures of acetone with other substances were effective. Mixtures of acetone with anethole ethyl salicylate and methyl salicylate gave normal solutions, notwithstanding that these substances when used alone appear to produce colloidal sols.

For use with plastics it was found that the most effective composition of the solution depended on the temperature to which the plastic could be safely heated before treatment and the following compositions were found to be satisfactory Table III Ester additive (60 parts to Quenching temperature, C. 40 to acetone) 210 Ethyl succinate. 200 Methyl salicylate. 180-200 Ethyl salicylate. l50200 Ethyl benzoate. 120-150 Iso-propyl acetate. -150 n-Butyl formate. Room temperature Acetaldehyde.

In each case the solution consisted of 0.2 gram of palladium chloride formed into the hydrochloric acid complex and dissolved in 40 parts of acetone and 60 parts of the ester.

The method of preparation of the methyl salicylate solution will be described by way of example.

0.2 gram of palladium chloride were weighed into a 200 ml. beaker, two to three ml. of concentrated hydrochloric acid were added, and the palladium chloride dissolved by warming. Heating was continued until the liquid was concentrated to a :brown syrup which was then allowed to cool.

To this syrup 50 ml. of pure acetone was added with stirring.

A mixture of 600 ml. methyl salicylate and 350 ml. acetone was prepared in a 2-litre beaker and very vigorously stirred with a mechanical stirrer, while the palladium chloride-acetone concentrate was slowly added.

If the excess acid is objectionable it may be neutralized by the gradual addition of small quantities of sodium bicarbonate, the solution being filtered afterwards.

The other mixtures referred to above were prepared in an exactly similar manner, the appropriate ester being substituted for the methyl salicylate.

The method of plating an article using pretreatment with the catalyst solution containing methyl salicylate prepared as described above will now be described by way of example.

This solution may usefully be employed for articles of the more heat resistant materials such as higher temperature stoving epoxy resin varnish films, glass, and ceramics and a pretreatment temperature of 200 C. is suitable with such a solution.

The object, which may be of any of these materials or any other material which will withstand heating to 200 C., is first thoroughly cleaned by chemical cleaning, etching or vapour blasting as may be appropriate.

The article is then heated to a temperature of 200 C., immersed in the 60% methyl salicylate-acetone solution prepared as described above, removed and dried in an oven at a temperature of 200 C. The process is then repeated, a total of 2-6 treatments being given. Since the article is above the temperature at which the acetonemethyl salicylate mixture begins to boil, boiling takes place at the surface on immersion, producing a quenching action. At each treatment the article is allowed to stand in the solution for a minute or so if it is porous, and for a longer period if it is non-porous.

The article is then immersed in a nickel plating solution of which the following is a typical example:

Sodium pyrophosphate grams/litre 50 Nickel sulphate do 25 Sodium hypophosphite do 25 Ammonium hydroxide, sufficient to maintain pH value between 10.0 and 11.0.

The temperature of the solution is maintained between 65 and 75 C.

If, instead, it is desired to plate the material with cobalt the procedure is similar, but the plating bath may be, for example, of the following composition:

Sodium hypophosphite grams/litre Sodium citrate do 20 Cobalt chloride do Ammonium chloride do 50 Ammonium hydroxide, sufiicient to maintain pH value The temperature of the solution is maintained at 95 C.

An essentially similar sequence of operations is used for plating other materials with the other solutions given above, except for the difference in the composition of the solution and the temperature at which the heat treatment is carried out.

In a further example, a sheet of plastic material which will not withstand temperatures much in excess of 110 C. is to be plated with phospho-nickel.

The palladium chloride complex is prepared as above, evaporated to a syrupy consistency, and dissolved in 50 ml. of acetone. This solution is then added, with stirring, to a mixture of 600 ml. of n-butyl formate and 350 ml. of acetone in a Z-litre beaker.

The plastic sheet is then pre-heated to a temperature of 110 C. and immersed in the catalyst solution for several minutes, then dried off in an oven at 110 C.

These steps are repeated 2-6 times, and the sheet then plated in the nickel plating solution given above.

It may be noted that the following liquids will not dissolve the palladium chloride complex:

Solvent Chemical classification Petroleum ether Aliphatic hydrocarbon. Benzene Aromatic hydrocarbon. Cyclohexane Alicyclic hydrocarbon. Chloroform Halogenated aliphatic hydrocarbon. Nitrobenzene Aromatic nitro-compound. Chlorobenzene Halogenated aromatic hydrocarbon. Di-iso-propyl ether Aliphatic ether.

However, certain of these, for example, benzene, chloroform, chlorobenzene and nitrobenzene can yield a solution when mixed With acetone. These solutions will also function as catalysts.

Similar complexes can be prepared from palladium chloride by using hydrobromic or hydriodic acids, instead of hydrochloric acid. It is to be noted, however, that the hydriodic acid complex prevents plating on copper treated with its solutions. This is thought to be due to the formation of a film of cuprous iodide on the surface of the copper.

To prepare a catalyst solution containing free hydrofluoric acid, suitable, for example, for nickel plating silicon Wafers in the manufacture of semi-conductive devices, or for nickel-plating tungsten rods which are required to be soldered, a catalyst solution containing hydrogen fluoride is used.

The complex of palladium chloride with hydrogen chloride and water is prepared as above and evaporated to a syrupy consistency by driving off excess water and hydrogen chloride. The syrupy liquid so formed is then dissolved in a small quantity of acetone, and there is added to it with stirring a quantity of an aqueous solution of hydrogen fluoride amounting to about 5% of the calculated quantity of the solution. The catalyst solution is then made up to the calculated quantity by adding acetone.

What is claimed is: l

1. In a chemical reduction plating process, the step of treating the surface to be plated before immersion in the plating bath with a catalyst comprising a solution of a complex of palladium chloride dissolved in an organic solvent, said complex comprising palladium chloride with hydrogen halide and Water evaporated to a condition not dryer than a paste.

2. The step defined in claim 1 in which the hydrogen halide is hydrogen chloride.

3. The step defined in claim 1 for treating the surface of a refractory material to be plated before immersion in the plating bath, said catalyst containing hydrogen fluoride in a range from about 5 to about 10% by volume of said catalyst.

4. A method of preparing a catalyst solution for use in a chemical reduction plating process comprising forming a complex of palladium chloride with water and an excess of hydrogen halide, evaporating off excess water and hydrogen halide to a condition not dryer than a paste and dissolving said complex in an organic solvent.

5. A method according to claim 4 in which the hydrogen halide is hydrogen chloride.

6. A method according to claim 5 in which hydrogen fluoride is added to said complex in an amount from about 5 to about 10% by volume of said total complex.

References Cited UNITED STATES PATENTS l/1963 Lukes 117-47 10/1965 Tsu et al. 117-47 X OTHER REFERENCES RALPH S. KENDALL, Primary Examiner. 

